Lead: A team led by PhD student Ewoud Wempe at the Kapteyn Institute (University of Groningen) reported on 5 March 2026 that a vast, flattened concentration of matter surrounds the Local Group. Using cosmological simulations initialized from the cosmic microwave background, the group produced a “virtual twin” of our neighborhood that reproduces the locations and motions of nearby galaxies. The model shows a broad sheet of ordinary and dark matter extending tens of millions of light-years, with large voids above and below the plane. That configuration explains why most large neighbors—aside from Andromeda, which approaches at ~100 km/s—appear to be receding rather than being drawn inward by the Local Group’s gravity.
Key Takeaways
- The study (reported 5 March 2026) used simulations seeded by CMB measurements to evolve a Local Group analog from the early universe to the present.
- The simulated environment reproduces the masses, positions, and velocities of the Milky Way, Andromeda, and 31 galaxies just outside the Local Group.
- Researchers identify a flattened matter distribution—a “cosmic sheet”—stretching tens of millions of light-years that contains both baryonic and dark matter.
- The sheet’s distributed mass counterbalances the Local Group’s pull, producing outward motions for galaxies that otherwise might be expected to fall inward.
- Above and below the plane the simulations show large cosmic voids, explaining the absence of inbound galaxies from those directions.
- The authors describe the result as the first detailed local dark-matter and mass-distribution model consistent with standard cosmology and local galaxy dynamics.
Background
Nearly a century after Edwin Hubble’s discovery that most galaxies recede from us—evidence for cosmic expansion—astronomers have long known local deviations exist. The nearest large neighbor, the Andromeda Galaxy, is a clear exception, moving toward the Milky Way at roughly 100 kilometers per second. For roughly fifty years researchers have been puzzled that many large galaxies near the Local Group nevertheless exhibit net recession instead of being drawn inward by the combined mass of the Milky Way, Andromeda and their satellites. That contrast between expected gravitational attraction and observed motions has been a persistent local cosmology puzzle.
Past attempts to explain local flows invoked measurement errors, peculiar velocities from past interactions, or atypical dark-matter clumping, but none reproduced the observed pattern in detail. The Local Group sits inside the cosmic web, where filaments, sheets and voids shape galaxy motions on tens to hundreds of millions of light-years. Mapping the local dark-matter distribution is challenging because dark matter is detected indirectly through gravitational effects, requiring combined observational constraints and numerical modeling. The new simulation-based approach addresses those challenges by generating many possible early-universe seeds and selecting realizations that match present-day observables.
Main Event
The international team led by Ewoud Wempe began with density perturbations constrained by cosmic microwave background data and ran high-resolution N-body and hydrodynamical simulations forward to z≈0. From that ensemble they selected realizations that reproduce the Milky Way and Andromeda masses and the observed positions and velocities of 31 galaxies just outside the Local Group. Because the chosen runs match multiple independent observables, the authors call the best realization a “virtual twin” of our cosmic neighborhood.
In those selected models the matter surrounding the Local Group is not isotropic but concentrated in a broad, flattened structure. This sheet spans tens of millions of light-years and contains both baryonic matter and the dark matter halos that dominate mass on these scales. Regions above and below the plane are comparatively empty, forming large cosmic voids with few galaxies. When the full mass distribution—including distant mass in the plane—is included, the net gravitational influence on nearby galaxies explains why many move away from the Local Group despite its local mass concentration.
The simulations reproduce not only radial speeds but the spatial pattern of galaxy motions: within the plane, the distributed mass offsets inward pull from the Milky Way–Andromeda pair, while outside the plane the scarcity of matter leaves galaxies receding under cosmic expansion. The team tested variations of the sheet’s mass and thickness and found the outward motion pattern is robust across a range of plausible parameters. The result does not require new physics beyond the standard ΛCDM framework; rather, it emphasizes how large-scale anisotropic mass distributions shape local dynamics.
Analysis & Implications
If the simulated sheet accurately reflects reality, it provides the first concrete local map of how dark matter and baryons conspire to set nearby galaxy motions. That has immediate consequences for interpreting peculiar velocities: what might look like anomalous outward motion can instead be a natural outcome of nearby mass arranged in a plane. The finding reduces tension between local dynamics and the global ΛCDM model by showing a configuration in which both are consistent.
For cosmologists, the result offers a new way to test structure-formation models at very small (tens of millions of light-years) scales. Simulations that incorporate the same initial conditions but different resolution or baryonic physics can be compared to the reported virtual twin to probe sensitivity to feedback, halo assembly bias, and dark-matter clustering. If independent groups reproduce similar sheet-like mass distributions from the same observational constraints, confidence in the interpretation would rise.
Observationally, the study motivates targeted surveys of galaxy distances and peculiar velocities in the Local Volume, and deep mapping of low-surface-brightness features that trace baryons in the proposed plane. Improved measurements of galaxy proper motions and weak-lensing mass maps around the Local Group could directly test the sheet hypothesis. Over time, corroborating or contradicting evidence will refine estimates of the sheet’s mass, thickness and extent and potentially tighten constraints on dark-matter properties.
Comparison & Data
| Quantity | Reported Value | Notes |
|---|---|---|
| Andromeda approach speed | ~100 km/s | Observed radial velocity toward Milky Way |
| Compiled nearby galaxies matched | 31 galaxies | Just outside the Local Group, positions & velocities reproduced |
| Sheet extent | Tens of millions of light-years | Flattened distribution spanning the Local Volume |
The table summarizes the principal numerical anchors used in the study and reproduced by the simulations. Matching 31 external galaxies gives the model discriminating power: many alternative mass configurations that fit fewer constraints would be less convincing. The sheet’s scale—tens of millions of light-years—is large enough to influence dynamics across the Local Volume but small compared with supercluster scales, placing it between halo and cosmic-web domains. These intermediate scales are precisely where baryonic physics and dark-matter distribution both matter for observed flows.
Reactions & Quotes
Lead author Ewoud Wempe framed the work as an attempt to enumerate local initial conditions that could produce our present-day neighborhood, emphasizing the dual consistency with cosmology and local dynamics. He highlighted the role of the sheet in reconciling apparent contradictions between expected gravitational attraction and observed recession of nearby galaxies.
“We explored all possible local configurations of the early universe that could lead to the Local Group, and we now have a model consistent with both cosmology and local motions.”
Ewoud Wempe, Kapteyn Institute (lead researcher)
Amina Helmi, an astronomer working on Galactic dynamics, welcomed the result as progress on a decades-old puzzle, noting that the study shows mass distribution can be inferred from galaxy motions alone. She emphasized that deriving a mass configuration consistent with observed positions strengthens the argument that the phenomenon is physical rather than observational noise.
“Based purely on the motions of galaxies, we can determine a mass distribution that matches positions within and just outside the Local Group.”
Amina Helmi (astronomer)
Unconfirmed
- Whether the specific sheet geometry in the selected simulation is unique; other realizations might produce similar outward motions with different mass distributions.
- The precise mass, thickness and radial extent of the sheet remain model-dependent until independent observational mass-mapping (e.g., weak lensing) confirms them.
- Observational verification of low-surface-brightness baryonic tracers aligned with the sheet has not yet been reported.
Bottom Line
The study provides a plausible, simulation-based explanation for why most large galaxies near the Local Group recede despite the pronounced gravity of the Milky Way–Andromeda system. By demonstrating that a broad, flattened mass distribution—including dark matter—can counterbalance local pull, researchers reconcile local galaxy motions with standard cosmology without invoking new physics. The result reframes a decades-old local puzzle as a consequence of anisotropic mass on intermediate cosmic scales rather than a contradiction of ΛCDM.
Key tests lie ahead: improved distance and velocity catalogs for Local Volume galaxies, targeted weak-lensing and deep imaging to map mass and baryons, and independent simulation groups attempting to reproduce the virtual twin. If those efforts corroborate the sheet, the finding will sharpen our picture of the Local Group’s environment and improve how peculiar velocities are used to probe cosmology and dark matter.